Weak. I have used rhyming dictionaries in moments of desperation, because I structure things according to the sonic flow of syllables, but generally it's better to work with subtler slant rhymes and so on.

I mean, this tool is fine, but aren't there rhyming dictionaries already all over the internet?

This is a really cool website design and I could see it totally helping out someone just starting to get into writing. That said, I probably won't use it that much because I'm trying to get my freestyle game up a few levels.